Method and apparatus for enhancing in-situ gas flow measurement performance

Abstract

An in-situ gas flow measurement controller measures the temperature and rate of pressure drop upstream from a flow control device (FCD). The controller samples the pressure and temperature data and applies the equivalent of a decimating filter to the data to produce filtered data at a slower sampling rate. The controller derives timestamps by counting ticks from the sampling clock of the A/D converter that is sampling the pressure at regular intervals to ensure the timestamps associated with the pressure samples are accurate and do not contain jitter that is associated with software clocks. The controller additionally normalizes the temperature reading to account for power supply fluctuations, filters out noise from the pressure and temperature readings, and excludes data during periods of instability. It calculates the gas flow rate accounting for possible non-linearities in the pressure measurements, and provides the computed gas flow measurement via one of many possible interfaces.

1. A method of computing rate at which gas flows out of a processing chamber gas stick, the method comprising:
 stopping the flow of gas into the gas stick at a first location which is upstream of a second location where gas is flowing out of the gas stick, 
 obtaining a plurality of measurements of the gas in a volume in fluid communication with, and located in between, said first and second gas stick locations, 
 deriving time-stamps for said measurements, 
 performing a gas flow calculation, where the gas flow calculation includes using at least two measurements and their corresponding derived time-stamps to compute the rate of gas flow out of said volume by calculating the rate of decrease in moles of the gas in the volume using a derivative of ideal gas law, 
 and providing results of the gas flow calculation via a system interface. 
 
     
     
       2. The method in  claim 1 , where the time-stamps are derived from a software clock. 
     
     
       3. The method in  claim 1 , where the time-stamps are derived from a hardware oscillator. 
     
     
       4. The method in  claim 1 , where the timestamp of a measurement is derived by adding a number to the time-stamp of previous measurement, where said number is an estimate of time elapsed since the previous measurement was sampled. 
     
     
       5. The method in  claim 4  where said estimate of elapsed time is computed by counting ticks on a hardware oscillator. 
     
     
       6. The method in  claim 5  where said hardware oscillator is the sampling clock of the ADC that is sampling the measurements. 
     
     
       7. The method in  claim 4  where a first measured sample at the beginning of a measurement period is set to the system time of when the sample was received. 
     
     
       8. The method in  claim 1 , wherein said measurements are taken from a pressure sensor. 
     
     
       9. The method in  claim 1 , wherein said measurements are taken from a temperature sensor. 
     
     
       10. The method in  claim 1 , wherein providing the results of the gas flow calculation via a system interface comprises sending an analog voltage signal over a conductor. 
     
     
       11. The method in  claim 1 , wherein providing the results of the gas flow calculation via a system interface comprises sending a digital signal over a digital interface. 
     
     
       12. The method in  claim 1 , wherein providing the results of the gas flow calculation via a system interface comprises storing a file on a computer-readable medium. 
     
     
       13. A method of computing rate at which gas flows out of a processing chamber gas stick, the method comprising:
 stopping the flow of gas into the gas stick at a first location which is upstream of a second location where gas is flowing out of the gas stick, 
 obtaining a plurality of measurements of the gas in a volume in fluid communication with, and located in between, said first and second gas stick locations, 
 obtaining compressibility factors of the gas that correspond to values of said measurements, 
 scaling said measurements by their corresponding compressibility factors, 
 performing a gas flow calculation, where the gas flow calculation includes using at least two scaled measurements to compute the rate of gas flow out of said volume by calculating the rate of decrease in moles of the gas in the volume using a derivative of ideal gas law, 
 and providing results of the gas flow calculation via a system interface. 
 
     
     
       14. The method in  claim 13 , where the compressibility factor for a measurement is obtained from a lookup table. 
     
     
       15. The method in  claim 13 , where the compressibility factor for a measurement is obtained by computing a value using a mathematical formula. 
     
     
       16. The method in  claim 13 , where a linear fitting formula is used to compute said rate of decrease in moles. 
     
     
       17. The method in  claim 13 , wherein said measurements are taken from a pressure sensor. 
     
     
       18. The method in  claim 13 , wherein said measurements are taken from a temperature sensor. 
     
     
       19. The method in  claim 13 , wherein providing the results of the gas flow calculation via a system interface comprises sending an analog signal over a conductor. 
     
     
       20. The method in  claim 13 , wherein providing the results of the gas flow calculation via a system interface comprises sending a digital signal over a digital interface. 
     
     
       21. The method in  claim 13 , wherein providing the results of the gas flow calculation via a system interface comprises storing a file on computer-readable medium. 
     
     
       22. A gas flow measurement system coupled to a gas stick in fluid communication with a processing chamber, the gas stick comprising a flow control device on the gas stick located upstream of the processing chamber, a valve, on the gas stick, located upstream of the flow control device, which has ability to regulate flow of gas through the gas stick, and a volume in fluid communication with, and located in between, the valve and the flow control device,
 the system comprising:
 one or more sensors in fluid communication with, and located in between, the valve and the flow control device, and, 
 a computer, in electronic communication with said sensors, and which comprises:
 a processor, 
 a computer-readable medium having stored thereupon a program that, when executed on said processor causes the processor to perform the steps: 
 stop the flow of gas into the gas stick by turning off the valve, 
 obtain compressibility factors of the gas that correspond to values of measurements, 
 scale said measurements by their corresponding compressibility factors, 
 perform a gas flow calculation, where the gas flow calculation includes using at least one scaled measurement to compute rate of gas flow out of said volume by calculating the rate of decrease in moles of the gas in the volume using a derivative of ideal gas law, 
 
 
 and provide results of the gas flow calculation via a system interface.